As earlier technology of this kind, there is a system arranged to plot a point corresponding to an actual wheel slip rate and a road surface friction coefficient in a two-dimensional map having a horizontal axis representing the wheel slip rate and a vertical axis representing the road surface friction coefficient, and to estimate the tire friction state from the slope of a straight line passing through the plotted point and the origin (cf. patent document 1). In accordance with the estimated tire friction state, this system controls the longitudinal force or driving/braking force of the wheel.    Patent Document 1: JP2006-34012 A